1. Field of the Invention
This invention relates generally to light transmitting optical fiber waveguides, and more particularly to a coupling apparatus for optically coupling a large cross-section high energy laser beam into a small diameter optical fiber waveguide.
2. Description of the Related Art
Blood vessel patency is very important to good cardiovascular health and can be severely reduced by deposits of cholesterol, fibrous material, and calcium on the inner surface of blood vessels such as arteries. These deposits are referred to as atherosclerotic plaque. If the plaque accumulates at one location, it can form an obstruction and drastically narrow the lumen of the artery, restricting and even totally blocking the flow of blood through the artery. Heart attack, stroke, and death can follow.
Current techniques and clinical applications make use of high energy laser beams to irradiate plaque until it is "vaporized," or changed into a form that is either removed from the body or harmlessly carried away in the bloodstream. This procedure is known as laser angioplasty, and involves inserting an optical fiber into a blood vessel, placing the distal end of the optical fiber adjacent the plaque obstruction, and conducting laser energy through the fiber to direct it at the plaque. Conventional angiography or angioplasty catheter insertion techniques are used to insert the optical fiber into the blood vessel. This form of relatively non-invasive laser microsurgery can eliminate the need for more radical surgical procedures, such as coronary artery bypass surgery. Once the laser catheter is in position, the obstruction can be vaporized by the laser energy. Because the inside diameter of a blood vessel can be very small (1 or 2 mm) in diameter and very tortuous in pathway, the optical fibers carrying the laser energy must be very small in diameter and extremely flexible. Typical fibers used in this application are made of fused silica or quartz and range in diameter from 50 to 1000 um.
Both continuous-wave (CW) lasers and pulsed, high energy lasers have been used to provide the vaporizing laser energy. The goal of the laser is to remove the plaque with laser energy without creating any thermal or mechanical injury to the healthy part of the blood vessel. CW, visible, and infra-red laser beams can often remove plaque, but they also have a side effect of creating severe injury (that is, burning) to the tissue directly adjacent to the lased area, whether healthy or not. There is, however, a type of laser that does not have the aforementioned problematic side effects, namely the excimer laser.
The excimer laser is a pulsed, high energy ultraviolet laser that can remove tissue by a non-thermal process known as photodecomposition, such as that described in U.S. Pat. No. 4,754,135. This non-thermal process can efficiently remove plaque without any thermal or mechanical injury to the adjacent healthy tissue. The most common excimer lasers are those that use rare-gas/halides (RGH) such as argon-fluoride, krypton-chloride, krypton-fluoride, xenon-chloride, or xenon-fluoride as the laser medium. With these RGH laser media, the laser energy pulses typically last on the order of tens to hundreds of nanoseconds in time, with an energy level ranging from a few millijoules to several joules per pulse.
In order to be useful, the laser beam must be efficiently coupled into the small diameter optical fiber that transmits the laser energy to the target atherosclerotic plaque. The coupling of the beam to the optical fiber is typically accomplished by a set of convex and/or concave focusing lenses. Because it is desirable to couple the largest fraction of total laser energy into the small fiberoptic, the input surface of the optical fiber is typically subjected to a beam of very high energy density. In this coupling process, great care must be taken not to exceed the laser-energy-density damage threshold of the fiber material. For fused silica fibers in the ultraviolet part of the spectrum this value is typically in the range of 50 to 100 mj/mm.sup.2. If the damage threshold is exceeded, catastrophic failure of the fiber can occur.
Because of the manner in which it is usually generated, the beam from an excimer laser is typically square or rectangular in cross-section, and its energy density is usually spatially very non-uniform. That is, in cross-section, the beam contains local areas of greater and lesser energy density, generating so-called "hot spots" in the beam. When the laser beam is focused onto the very small surface area of an output fiberoptic, even though the average spatial energy density of the beam might be below the fiber's damage threshold, the peak spatial non-uniformities, or hot spots, can easily exceed the damage threshold. The energy density of the hot spots can be great enough to cause catastrophic damage to the optical fiber both at the fiber input surface where the beam enters the fiber and within the bulk of the fiber. Thus, the average energy density of the beam that may be safely used is often limited by the energy density of the hot spots. The focusing lenses used for directing the beam from the laser into the fiber do nothing to alleviate these non-uniformities. Therefore, in order to maximize the laser energy input to the fiber, and hence the output from the fiber, the energy density distribution of the laser beam entering the fiber must be completely free of all spatial hot spots.
A typical prior art laser-optical fiber coupling apparatus for laser angioplasty is described in U.S. Pat. No. 4,842,360 and is illustrated in FIG. 1. In the drawing, a high energy excimer laser 10 is coupled to a small diameter output optical fiber 12. The distal end of the fiber is inserted into a patient's blood vessel and advanced to the site of an arterial obstruction. The laser generates a beam 14 having a rectangular cross-section, as indicated at the point marked A. The beam is directed through an aperture plate 16 having a circular aperture 18 in its center. A cross-section of the aperture plate is shown at B. The aperture plate produces a beam 20 having a circular cross-section, as illustrated at the point labeled C. The beam is typically directed to a focusing lens 22. This lens focuses and concentrates the beam into a smaller diameter beam 24 having a reduced circular cross-section. The beam then enters the end of the small diameter optical fiber 12 at a fiber input surface 26, where it will be conducted to the desired point. The input surface is typically located from the lens at a distance slightly greater than the focal length of the lens, in order to prevent the beam from converging inside the fiber and damaging the fiber from excessive laser energy density. The beam cross-section is shown at the point marked D. Alternatively, an additional lens may be inserted between lens 22 and the input surface of the fiber 26 in order to collimate the laser beam as it enters the fiber 12. If the laser beam has hot spots anywhere within the beam cross-section that exceed the damage threshold, the input surface of the optical fiber can become scarred, pitted, or fused. The optical fiber would then be incapable of effectively transmitting the laser energy.
There are several drawbacks to the FIG. 1 type of apparatus. First, the aperture plate 16 geometrically obscures anywhere from 12% to 60% of the beam 14 energy by eliminating the squared corners of the excimer laser beam 14. Of course, this is a tremendous waste of the energy generated by the laser and requires the use of a laser much larger than would otherwise be needed if all the laser's energy could be coupled into the output optical fiber. Second, the apparatus does nothing to eliminate the spatial nonuniformities, or hot spots, generated in the beam and transmitted to the small diameter output optical fiber 12. In addition, such an apparatus requires careful optical alignment of all of the elements from the laser 10 to the input surface 26 at the small diameter optical fiber. Any misalignment can cause an energy loss at best and catastrophic damage to the optical fiber at worst. The optical fiber is typically a disposable item, and establishing optical alignment with the laser can be an exacting, time consuming process that must be performed every time a ne fiberoptic is connected to the laser.
Therefore, there is a need for a high energy laser-to-fiberoptic waveguide coupling technique and apparatus for coupling a high energy, non-uniform energy density laser beam to a small diameter optical fiber without allowing the beam to cause damage to the optical fiber. Thus, the apparatus should eliminate the damaging hot spots present in the beam. Such an apparatus also should maximize the laser beam transfer efficiency by utilizing substantially all of the beam generated by the high energy laser without discarding portions of the beam to obtain the desired circular cross-sectional shape.